This application is a 35 USC 371 application of PCT/DE 00/02735 filed on Aug. 12, 2000.
1. Field of the Invention
The invention relates to a fuel injection system and more particularly to an improved fuel injection system which produces two different injection pressures.
2. Description of the Prior Art
For the sake of better understanding of the description and claims, several terms will now be explained. The fuel injection system according to the invention can be embodied as either stroke-controlled or pressure-controlled. Within the scope of the invention, the term stroke-controlled fuel injection system will be understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable valve member as a result of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be effected by a final control element (or actuator). In a pressure-controlled fuel injection system according to the invention, the valve member is moved counter to the action of a closing force (spring) by the fuel pressure prevailing in the nozzle chamber of an injector, so that the injection opening is uncovered for an injection of the fuel from the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder is called the injection pressure, while the term system pressure is understood to be the pressure at which fuel is available or is stored inside the fuel injection system. Fuel metering means delivering fuel to the nozzle chamber by means of a metering valve. In combined fuel metering, a common valve is used to meter various injection pressures. In the pump-nozzle unit (PDE), also called a unit fuel injector, the injection pump and the injector form a unit. One such unit per cylinder is built into the cylinder head and driven either directly via a tappet or indirectly via tilting levers by the engine camshaft. The pump-line-nozzle system (PLD) operates by the same method. In this case, a high-pressure line leads to the nozzle chamber or nozzle holder.
A unit fuel injector is known for instance from German Patent Disclosure DE 195 175 78 A1. In this fuel injection system, the system pressure is generated via a piston that can be acted upon by pressure and whose motion is controlled by a cam drive. A variable fuel injection of different quantities for the sake of preinjection, main injection and postinjection is only limitedly feasible by means of this kind of fuel injection system.
To achieve fuel injection with the aid of a unit fuel injector or a pump-line-nozzle unit over a wide rpm range with great precision, a fuel injection system according to the invention is proposed. Refinements make it possible to remove pollutant exchange and more-flexible preinjection and optionally a postinjection by means of a unit fuel injector or a pump- line-nozzle system. If a valve with a cross sectional control, for instance by means of a piezoelectric actuator, is used for the fuel metering, then improved metering of the injected fuel quantity can be achieved. This creates a good minimum-quantity capability in the preinjection. The development of the injection course in the main injection can be varied in a targeted way. Each unit fuel injector or pump-line-nozzle unit can contain a pressure storage chamber, which can be decoupled from the unit and filled with fuel during the pumping stroke of the compression device. By means of the pressure storage chamber, control of the injection pressure can be done relatively independently of the engine rpm. The time between the triggering of the pressure buildup and the injection can be selected freely within wide ranges. The time of the onset of the pressure buildup determines the pressure level attained.